1. Technical Field
The present invention relates to a power unit capable of continuously performing a normal operation even when the voltage of a commercial power is varied and a lighting apparatus including the power unit.
2. Description of Related Art
Recently, lighting apparatuses using a light emitting diode (hereinafter referred to as an LED) as a light source have been developed for various purposes, and have being replaced lighting apparatuses using a conventional light source such as an incandescent lamp or a fluorescent lamp. Furthermore, as vehicle lighting apparatuses such as a headlight and a taillight, lighting apparatuses using an LED as a light source have already been commercially available.
In a power circuit (a power unit) used in a lighting apparatus using an LED as a light source, power control different from that employed in a lighting apparatus using a conventional light source is required because of a difference in characteristics between the conventional light source such as an incandescent lamp and the LED. For example, since an LED is required to drive with a constant current due to the electric characteristics of the device, a power circuit of a lighting apparatus using the LED as a light source should employ constant current output specifications with a voltage/current output characteristic for keeping a current constant against variation of a voltage instead of general constant voltage output specifications.
On the other hand, failure modes of an LED include two modes of an open mode and a short mode, and therefore, the power circuit should be provided with an overload protection circuit as a protection function against an overload state such as overcurrent or overvoltage derived from a failure or the like of the LED. The overload protection function is realized by, for example, detecting overcurrent of a switching element on a primary side of a switching transformer included in the power circuit so as to detect an overload state such as overvoltage or overcurrent of a load on a secondary side of the switching transformer, and thus, the power output is halted when the overload state is caused.
Moreover, an overcurrent protection device to be used in a power unit employing an inverter control method using an element other than a light source as a load has been disclosed (see Japanese Patent Application Laid-Open No. 4-217877), in which an abnormal detection signal is output from an overcurrent detection circuit when an input current to an inverter exceeds a prescribed value, and a latch circuit to be reset in accordance with the abnormal detection signal for keeping in the reset state until a reset signal is input is provided so as to continuously halt the output of the inverter.